Hyperthreaded, Dual Core or Quad Core

I am running a high traffic oscommerce website on a cpanel server. At some times, when hundreds of customers sign-in simultaneously, server becomes very slow or mysql goes down. So, we are planning to migrate to a new high end server. We are looking for a Dual Xeon server with 8GB RAM and SCSI hard disks, but I have following questions:-

1. Which processors will be better - Hyperthreaded, Dual Core or Quad Core?